`combine_slices` optimization

[jond01]

After inspecting in detail the combine_slices code, I found some optimization possibilities:

• _sort_by_slice_position is called twice:
  First in the voxels (_merge_slice_pixel_arrays),
  Second in the affine (_ijk_to_patient_xyz_transform_matrix).
  Obviously, it can be done once in the outer combine_slices function.
• _slice_positions is called 4 times:
  _validate_slices_form_uniform_grid - 1 time
  _slice_spacing - 1 time
  _sort_by_slice_position - 1 time, but _sort_by_slice_position is called twice --> 2 times
• _extract_cosines is called 6 times:
  _validate_image_orientation - 1 time
  _ijk_to_patient_xyz_transform_matrix - 1 time
  _slice_positions - 1 time, but _slice_positions is called 4 times --> 4 times

All these calls better be done once (and can be).
However,

1. this may incorporate substantial redesign of the code,
2. it will change the functionality of _merge_slice_pixel_arrays and _ijk_to_patient_xyz_transform_matrix, which should not be used alone, but there are some special tests for _merge_slice_pixel_arrays.

Originally posted by @jond01 in #16 (comment)